I do mine for an hour. Conversion probably completes within 45, but it doesn't hurt to go a little longer. Most of the color influence from the grain seems to happen in the first 10-15 minutes, so I don't think going longer makes it any darker.
One thing that might help with color (if you're not already doing this) is adding the extract late in the boil instead of the beginning. I shoot for 3 gallons of runoff between the mash and sparge, add only add about a cup of extract at the beginning and then put the rest of the extract in at the 10 minute mark (off heat while adding). I've been able to make some pretty light colored brews this way.